Traditional marine timbers and pilings are made of steel, concrete or wood. Steel and concrete are very heavy and expensive and do not have the desired resiliency for fendering applications. Steel is especially subject to rapid corrosion in a marine environment. Wood suffers from rapid erosion and is subject to attack by marine animals which reduce its effectiveness. To prolong its useful life, wood used for marine timbers and pilings, as well as for non-marine applications such as telephone poles, railroad ties and the like, is sometimes treated with a preservative, such as creosote. However, creosote and other preservatives are detrimental to the environment. Furthermore, given the recent efforts for preservation of forests, the use of wood timbers, pilings, poles and railroad ties is not desirable.
To a great extent, marine timbers and pilings made of plastic solve the foregoing problems with respect to steel, concrete and wood marine timbers and pilings. U.S. patent application Ser. Nos. 08/862,305 filed May 23, 1997, and U.S. Pat. Nos. 5,650,224 and 5,658,519, all of which are assigned to the assignee of the present invention and the subject matter of which is incorporated herein by reference, disclose methods and apparatus for manufacturing extruded plastic members. In the methods disclosed in the aforesaid application and patents, the continuously extruded members are reinforced with a plurality of substantially rigid bars, rods or rebars that are fed to the extrusion die continuously or in discrete lengths. The ends of each bar include joint connections for splicing the bars in end-to-end relation as described in the aforementioned U.S. patent application Ser. Nos. 08/862,305 filed May 23, 1997. The molten plastic entering the die surrounds and bonds to the plurality of reinforcing bars and is then cooled so that the molten plastic, the plastic core and the reinforcing bars are formed into a monolithic elongated member having a predetermined cross-section. The extruded plastic member is then cut into predetermined lengths at the locations of the splices or joint connections so that the reinforcing bars in the extruded member are joint or splice-free.
In the resulting extruded members, the plastic shell is intended to withstand the compressive stresses, and the reinforcing bars are provided for bearing the tensile and shear stresses. It has been found that undesirable internal stresses occur during the cooling of the molten plastic, resulting in the formation of cracks and the eventual failure of the extruded member. Accordingly, there is a need for methods of and apparatus for manufacturing elongated plastic members which relieve or eliminate the thermal and shrinkage stresses that result upon cooling the molten plastic about the reinforcing members. There is a further need for methods of relieving stresses in extruded members that are compatible with existing continuous extrusion processes and apparatus and that may be implemented prior to the final cutting of the member to length.